Criteria for Prolonged QT Interval
A prolonged QT interval is defined as a corrected QT (QTc) ≥460 ms in women and >450 ms in men, with the QT interval measured from the beginning of the QRS complex to the end of the T wave and adjusted for heart rate. 1
Gender-Specific Diagnostic Thresholds
The diagnostic criteria differ by sex due to physiological differences in ventricular repolarization:
- Women: QTc ≥460 ms is considered prolonged 1, 2
- Men: QTc >450 ms is considered prolonged 1, 2
- Short QT interval (both sexes): ≤390 ms 1, 2
The gender difference of 12-15 ms in younger adults narrows to 6-10 ms in older age groups and practically disappears after age 40. 1, 2
FDA Severity Classification for Drug Studies
When evaluating QT-prolonging effects of medications, the FDA recommends reporting three severity levels: 1, 2
- >450 ms (men) or >460 ms (women): Prolonged QT
- >480 ms: Moderate prolongation (borderline high risk)
- >500 ms: Severe prolongation (significantly increased risk of torsades de pointes) 3, 4
Pediatric Criteria
For children, the thresholds differ by age: 1
- Children <12 years: QTc ≥450 ms is considered prolonged (98th percentile)
- Ages 12-16 years: Gender differences begin to emerge (8 ms difference)
- Caution: Bazett's formula may produce false QT prolongations in children 1
Clinical Risk Stratification
The absolute QTc value correlates with arrhythmia risk: 2, 3, 4
- QTc 460-480 ms (women) or 450-480 ms (men): Borderline prolongation requiring monitoring
- QTc 480-500 ms: Moderate risk; avoid QT-prolonging medications 2, 5
- QTc >500 ms: High risk for torsades de pointes; discontinue offending agents, correct electrolytes, consider IV magnesium 5, 3, 4
Important Measurement Considerations
Rate Correction Methods
- Linear regression functions are preferred over Bazett's formula for QT-rate correction 1, 2
- Bazett's formula (QTc = QT/√RR) is most commonly used but has significant limitations, particularly at extreme heart rates 2, 6
- Fridericia's formula is more accurate, especially at higher heart rates 2
- Do not attempt rate correction when RR interval variability is large (e.g., atrial fibrillation) 1, 2
QRS Duration Adjustment
When ventricular conduction defects are present (QRS >120 ms): 1, 2
- The QT interval prolongs and requires adjustment for QRS duration
- Use the JT interval (QT duration minus QRS duration) with JT-specific normal standards
- Alternatively, incorporate QRS duration and RR interval as covariates into the adjustment formula
Serial Monitoring Thresholds
For drug-induced QT prolongation, clinically significant changes from baseline include: 1, 2, 5
- Increase >30 ms: Warrants closer monitoring
- Increase >60 ms: Clinically significant; consider dose reduction or discontinuation 1
Common Pitfalls to Avoid
- Automated measurements can be inaccurate, especially with abnormal baseline ECGs; manual verification is essential 6
- Bazett's formula overcorrects at heart rates >80 bpm and undercorrects at bradycardia 2, 7
- U-wave inclusion: Ensure the T-wave end is properly identified and U-waves are not included in the measurement 1
- Lead selection: Choose a lead with T-wave amplitude ≥2 mm and well-defined T-wave end 2
- Gender-neutral thresholds: Always apply sex-specific criteria, as using a single threshold misses prolongation in women and overdiagnoses in men 1, 2
High-Risk Clinical Scenarios
Certain conditions amplify the risk of torsades de pointes even at lower QTc values: 5, 6, 8